1. Field of the Invention
This invention relates to white light emitting devices with a high (typically ≧80) CRI (Color Rendering Index). More especially the invention concerns white light emitting devices based on solid state light emitting devices, typically LEDs (Light Emitting Diodes), and drive circuitry for operating such devices.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until high brightness LEDs emitting in the blue/ultraviolet (U.V.) part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more down converting (i.e. converts photons to photons of a lower energy) phosphor materials, that is photoluminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (longer wavelength). Typically, the LED chip generates blue light and the phosphor material(s) absorbs a proportion of the blue light and re-emits light of a different color, typically yellow or a combination of green and yellow light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor material provides light which appears to the eye as being nearly white in color.
Due to their long operating life expectancy (of order 30-50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources. Today, most lighting fixture designs utilizing white LEDs comprise systems in which a white LED (more typically an plurality of white LEDs) replaces the conventional light source component. Moreover, due to their compact size, compared with conventional light sources, white LEDs offer the potential to construct novel and compact lighting fixtures.
The ability of a light source to render the color of an object is measured using the Color Rendering Index (CRI) which gives a measure of how a light source makes the color of an object appear to the human eye and how well subtle variations in color shade are revealed. CRI is a relative measurement of the light source's ability to render color compared with a black body radiator. In applications where accurate color rendition is required, such as for example retail lighting, museum lighting and lighting of artwork, a high CRI (typically at least 80) is highly desirable.
A disadvantage of white LEDs can be their relatively low CRI, typically <75, compared with an incandescent source whose CRI>95. The low CRI is due to the absence of light in the red (>600 nm) part of the spectrum. To improve the CRI of a white LED it is known to incorporate a red light emitting phosphor material. However compared with yellow and green down converting phosphor materials, red light emitting phosphor materials have disadvantages. Firstly the energy loss associated with the phosphor material down converting blue light (450 nm, energy 2.76 eV) to red light (630 nm, energy 1.97 eV) is larger than that associated with converting blue light to yellow light (550 nm energy 2.25 eV). This is generally referred to as Stokes loss and the higher Stokes loss associated with red light emitting phosphor materials can reduce the luminous efficacy (lm/watt) of the source. Secondly, since the human eye is less sensitive to red light compared with green or yellow light this requires a larger quantity of red phosphor material to give an equal effect on the eye.
U.S. Pat. No. 6,513,949 and U.S. Pat. No. 6,692,136 teach hybrid white LED lighting systems comprising a combination of one or more LEDs (red or green) and a phosphor-LED consisting of a blue LED and at least one phosphor (green or amber).
U.S. Pat. No. 6,577,073 disclose an LED lamp that includes blue and red LEDs and a phosphor. The blue LED produces an emission falling within a blue wavelength range. The red LED produces an emission falling within a red wavelength range. The phosphor is photo-excited by the emission of the blue LED to exhibit photoluminescence having an emission spectrum in an intermediate wavelength range between the blue and red wavelength ranges.
U.S. Pat. No. 7,213,940 disclose a white light emitting device that comprises first and second groups of solid state light emitters (LEDs) which emit light having a dominant wavelength in a range 430 to 480 nm (blue) and 600 to 630 nm (red) and a phosphor material which emits light with a dominant wavelength in a range 555 to 585 nm (yellow).
Although using a red emitting LED can improve both luminous efficacy and CRI the inventors have appreciated that such a device has limitations. Most notably the Correlated Color Temperature (CCT) and CRI of light generated by such a device can vary significantly with operating temperature. As represented in FIG. 1a the change in emission intensity of blue and red light emitting LEDs with operating temperature is different. Typically the emission intensity of a red LED decreases much more quickly than a blue LED with increased operating temperature. For example over an operating temperature range of 25° C. to 75° C. the emission intensity of a GaN-based blue LED can decrease by about 5% whilst the emission intensity of a AlGaInP-based red LED can decrease by about 40%. In a white light device based on blue and red LEDs these different emission/temperature characteristics will, as shown in FIG. 1b, result in a change in the spectral composition of the emission product and hence an increase in CCT with increased operating temperature. As is known the CCT of a white light source is determined by comparing its hue with a theoretical, heated black-body radiator. CCT is specified in Kelvin (K) and corresponds to the temperature of the black-body radiator which radiates the same hue of white light as the light source. Moreover as shown in FIG. 1b a reduction in the relative proportion of red light in the emission product with increasing operating temperature will result in a decrease in CRI.
A need exists therefore for a high CRI white light emitting device based on solid state light emitters that at least in part overcomes the limitations of existing devices.